12.04 - Apt-get update always stuck at getting Release.gpg

While I run apt-get update on Ubuntu 12.04, I got the following output (partially):

Get:1985 http://archive.ubuntu.com precise Release.gpg [28.9 kB]
...

My sources.list file is not modified and I use the main server as the repository.

I tried to remove /var/lib/apt/lists/* and update again but failed.

I retrieved the Release.gpg file manually: http://archive.ubuntu.com/ubuntu/ubuntu/dists/precise/Release.gpg

Found that the file size is 198 instead of 28.9kB, any ideas?

Thank you very much!